1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a transfer object, by forming a visible image on a surface of a photosensitive element with electrically charged toner particles, and electrostatically transferring the visible image onto the transfer object.
2. Discussion of Related Art
As a kind of an image forming apparatus for forming an image on a transfer object such as a sheet of paper, there is known an apparatus having a photosensitive element, a charger for uniformly charging a surface of the photosensitive element at a voltage of a polarity, an exposure device for exposing the electrically charged surface of the photosensitive element to a light, so as to form an electrostatic latent image thereon, a developing device for forming a visible image (i.e. developer image) by depositing toner particles (i.e. developer) of a color on the electrostatic latent image formed by the exposure device, and a transferring device for transferring the developer image formed on the surface of the photosensitive element onto the transfer object.
There is known a problem encountered when the transferring device of such a kind of an image forming apparatus transfers the toner particles forming the developer image, onto the transfer object. Namely, some toner particles are electrostatically charged in a polarity reverse to a regular polarity in which the toner particles are charged to be attracted to the photosensitive element to form the visible or toner image corresponding to the latent image, and a part of such toner particles charged in the reverse polarity fails to be transferred onto the transfer object and is left as a waste toner on the photosensitive element after the transferring step.
The polarity of the charge of the waste toner on the photosensitive element is again reversed to the regular polarity, when the charger charges the surface of the photosensitive element for forming the next visible image (i.e., toner image) on the surface of the photosensitive element, and then the waste toner is transferred onto a transfer object by the transferring device. The waste toner reversely charged, that is, the waste toner charged in the polarity reverse to the regular polarity in which the toner particles are charged to form the visible toner image on the photosensitive element, is often left at locations on the photosensitive element where the next visible image is not to be formed, adversely affecting the next visible image.
In a case where the image forming apparatus is for color printing and constructed to sequentially form a plurality of toner images on the photosensitive element by using respective color toners, it may occur that a part of reversely charged particles of a toner of a first color is carried by and on the transfer object to a position to be brought into contact with a photosensitive element for forming a toner image of a second color, and thus retransferred to the photosensitive element for the second color.
The reversely charged toner particles retransferred onto the photosensitive element for the second color may adhere to, and intrude into, a developing device for the second color, which makes the second color mixed with the first color inside the developing device for the second color. In this case, the color of the toner image formed by the developing device for the second color is made different from the expected one.
To solve these problems, there has been proposed in JP-A-2000-242152 an image forming apparatus equipped with a cleaning member or roller, which is disposed to be opposed to a photosensitive element and removes a waste toner as reversely charged from a surface of the photosensitive element, by electrostatically attracting the waste toner.
Meanwhile, the waste toner adhering to the surface of the photosensitive element is not constituted solely by reversely charged toner particles, but regularly charged toner particles, i.e., toner particles charged in the regular polarity, are also included in the waste toner. The regularly charged waste toner particles have failed to be transferred to the transfer object, and remain on the photosensitive element even after the toner image is transferred onto the transfer object.
Having the same polarity as the cleaning roller, the regularly charged waste toner particles are not attracted to the cleaning roller, and thus can not be removed in the conventional apparatus.
To remove such regularly charged waste toner, there may be additionally provided another cleaning roller which is charged in a polarity suitable for attracting the regularly charged waste toner. In this case, however, the number of required components increases and the size of the apparatus accordingly increases.
There may be employed a method in which a cleaning plate or the like is pressed onto the photosensitive element to scrape off all waste toner particles irrespective of their polarity. However, since the photosensitive element is damaged by a use of such a cleaning plate, this method is not preferable.